This invention relates to inertial navigation systems, and more particularly, to torquing systems for inertial instruments.
Many forms of inertial systems require stable current pulses for application to linear torquers in order to provide highly stable, quantized levels of torque. In one form, the conventional torquing systems drive a torquer with a constant repetition rate stream of controlled width, constant amplitude pulses. In another form, the repetition rate is controlled for a stream of substantially uniform pulses.
For systems requiring fine torque quantization relative to the maximum torque level, the latter approach is generally utilized with the individual current pulses being relatively short and being applied at a high rate to the torquer. Typical conventional systems generate torque pulses by constructing a constant current source, and chopping the resultant current into constant amplitude pulses with high speed switches. The performance of torquing systems is generally expressed in terms of the electronics scale factor, or the "gain" of the torquing electronics. The scale factor is proportional to the area of the individual torque pulses, and relates the average torquing current to the average number of pulses per second. In high rate torquing systems, the relatively narrow and constant amplitude current pulses are often subject to variation from pulse to pulse, such as may be due to switching time variations of the current chopping switches. Such conditions result in an undesirable trade-off between scale factor stability and torquing rate.
Many of the prior art torquing systems, particularly, the high speed systems, utilize transistor switching circuits to accomplish the current chopping. However, in the applications for many such systems, the chopping switches are subjected to particularly hostile environments, such as a high neutron flux in the case of on-board guidance instrumentation for a missile during re-entry. Under such conditions, the gain of the current chopping transistors falls off substantially, leading to corresponding variations in switching times. As a result, the width of the chopped current pulses varies from its nominal value with the instability in the pulse width causing a corresponding instability in the scale factor.
Furthermore, in the prior art systems, the pulse width control is generally achieved through the use of a model of the torquer current. The modeling typically introduces the very type of switching errors described above, since the transistors in the model circuit also suffer degradation in certain environment. In addition, the model circuit generally includes an integrating network which introduces a first order scale factor sensitivity to the gain of the integrator, principally due to non-linear capacitance and inductive variances with time and temperature.
It is an object of the present invention to provide a network for generating control pulses for an inertial instrument torquer characterized by constant current-time integral, or area.